1. Field of the Invention
The present invention relates to brake boosters, in particular for motor vehicles, according to the preamble of claim 1, such as are known e.g. from WO 99/26826. Such brake boosters generally take the form of vacuum brake boosters and are used during braking to provide the driver of a vehicle with an auxiliary force so that the brake actuating force to be summoned up by the driver may be kept to a comfortable level. Purely hydraulically operating brake boosters are also known and the present invention is therefore not restricted to vacuum brake boosters.
2. Description of Related Art Including Information Described Under 37 CFR 1.97 and 1.98
Irrespective of the manner in which the auxiliary force is generated, e.g. by means of a vacuum or hydraulically, a brake booster usually has a control valve for controlling the boosting force it generates, i.e. the amount of auxiliary force, as well as a control valve housing. Extending at least partially into the control valve housing is an input element, with the aid of which a braking request of the driver is communicated to the brake booster. The input force, which is introduced via the input element into the brake booster, and the auxiliary force, which is subsequently generated by the brake booster, are combined at an output element and delivered by the output element to a master cylinder, connected downstream of the brake booster, of a vehicle hydraulic brake system. If the brake booster is a vacuum brake booster, the output element generally comprises a rubber-elastic material in disk form, which is disposed in a chamber in an end piece of the control valve housing and behaves like a liquid. Situated between the output element and the input element is a thrust piece, which is movable axially relative to the control valve housing. The actuating force introduced into a brake booster therefore flows via the input element and from there, optionally via interposed components such as valve pistons or the like, to the thrust piece and then to the output element.
It has already been known for some time that most drivers of a motor vehicle in an emergency braking situation do not actuate the brake strongly enough. This behaviour is disadvantageous particularly if the vehicle brake system is equipped with an anti-skid system, because the maximum braking action of a brake system equipped with an anti-skid system may be achieved only when each vehicle wheel during braking enters a slip-controlled state, i.e. when each vehicle wheel is braked to such an extent that the slip control of the anti-skid system is activated. This state is reached only when a sufficiently high hydraulic actuating pressure is supplied to each vehicle wheel, this in practice frequently not being the case.
As a solution to this problem, a device mostly described in technical literature as “brake assistance” is proposed. To put it concisely, this brake assistance ensures that in an emergency braking situation, i.e. when the input element is moved very quickly a relatively long way in the actuating direction, the brake booster provides its maximum auxiliary force. Early brake assistance solutions employed an electromagnet which, after identification of an emergency braking situation, independently of the actual input force held the air control valve of a vacuum brake booster in the open position so that the maximum pressure difference between a vacuum chamber and a working chamber and hence the maximum possible auxiliary force was able to build up in the vacuum brake booster. In order to achieve the same effect without an expensive electromagnet, later solutions propose that the previously mentioned thrust piece be supported in an emergency braking situation against the control valve housing so that the hydraulic reaction forces transmitted from the master cylinder back into the brake booster do not retroact upon the input element of the brake booster and hence upon the brake pedal but are taken up by the brake booster. Thus, with a relatively low input and/or actuating force a high output force may be achieved, this being desirable in an emergency braking situation. Solutions of the last-described type are known from the previously mentioned WO 99/26826 and from EP 0 901 950 B1.
The last-mentioned solutions are however mechanically relatively complex and therefore not much less expensive than the likewise mentioned electromagnetic solution.